1. Field of the Invention
The present invention relates to angiography wherein a two dimensional array of pixels for display as an angiogram is determined from a three dimensional array of voxels computed from signal samples of radiation due to flowing blood in a region of a body under examination. In its particular aspects, the present invention relates to interpolation methods of providing an increased apparent resolution in the angiogram.
2. Background of the Invention
Display algorithms for rendering three-dimensional Magnetic Resonance Angiography (MRA) data in two-dimensional form are known from H. Cline et al, "Volume Rendering and Connectivity Algorithms for MR Angiography" Magn. Res. Med. 18, pp. 384-394 (1991).
Angiography of the type mentioned is presently carried out by operating magnetic resonance imaging (MRI) apparatus in a mode employing a magnetic resonance angiography (MRA) technique in which a three dimensional data set of voxels exhibiting enhanced contrast to flowing blood is post-processed to display the vascular area of interest. Time-of-flight techniques for both 2D and 3D collections and 3D phase-contrast techniques are known for enhancing the contrast of flowing blood relative to stationary tissue. In the 2D time-of-flight method, a collection of spin resonance signals for multiple parallel slices is obtained. The flow sensitive contrast is due to substantially saturating stationary spins in the slice from which spin resonance signals are collected by relatively rapid repetition of close to 90.degree. flip angle slice selective RF excitation pulses so that only unsaturated spins in blood flowing into the slice have relatively strong longitudinal magnetization just prior to the excitation pulses. This induces high intensity spin resonance signals from the inflowing blood, which intensity increases with the amount of inflow velocity component normal to the slice. A three dimensional data set of voxel intensities is computed by two dimensional Fourier transformation for each slice of samples of the spin resonance signals received during a read gradient for sequences repeated with different phase encoding gradient integrals. In the 3D time-of-flight method and in the 3D phase-contrast method a three dimensional data set of spin resonance samples are obtained from which the three dimensional data set of voxel intensities is obtained by three dimensional Fourier transformation.
Irrespective of the technique employed to obtain the three dimensional data set of voxel intensities, a rendering of this data set for viewing purposes to a two dimensional data set of pixels is necessary. This is done typically by forming a projection in a viewing direction. The most widely used projection method is maximum intensity projection (MIP). With this method, which is computationally fast, parallel rays are projected through the three dimensional data set in a viewing direction, a different ray being associated with each pixel, and the maximum intensity of voxels along each each ray is taken as the intensity of the associated pixel.
If the center to center spacing in one of three mutually orthogonal directions, e.g. between adjoining voxels in different slices of a multislice collection, is larger than that between adjoining voxels in the other directions, it is common to provide additional apparent resolution in the slice direction by interpolation. Interpolation is usually applied prior to projection increasing the number of planes of voxels in order to produce an expanded three dimensional data set with substantially cubic voxels. Alternatively, interpolation can be applied to the two dimensional pixel array produced by projection to expand the number of lines quantizing the slice direction.
Both interpolation techniques produce an objectional staircase artifact along the boundaries between flowing blood and stationary tissue when blood vessels are oriented obliquely with respect to the principal directions of the voxel spacings. This is because conventional interpolation techniques utilize a constant direction of interpolation which is aligned with one of the principle axes of the voxel array.